# std::regex_traits<CharT>::isctype

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< cpp‎ | regex‎ | regex traits

bool isctype( CharT c, char_class_type f ) const; |  |

Determines whether the character c belongs to the character class identified by f, which, in turn, is a value returned by lookup_classname() or a bitwise OR of several such values.

The version of this function provided in the standard library specializations of std::regex_traits does the following:

1) First converts f to a value m of type std::ctype_base::mask.
For each std::ctype category listed in the table in the page lookup_classname(), if the bits in f corresponding to the category are set, the corresponding bits in m will also be set.
2) Then attempts to classify the character in the imbued locale by calling std::use_facet<std::ctype<CharT>>(getloc()).is(m, c).

- If that returns true, `isctype()` will also return true.
- Otherwise, if c equals '_', and f includes the result of calling lookup_classname() for the character class `[:w:]`, true is returned, otherwise false is returned.

### Parameters

c | - | the character to classify
f | - | the bitmask obtained from one or several calls to lookup_classname()

### Return value

true if c is classified by f, false otherwise.

### Example

Run this code

    #include <iostream>
    #include <regex>
    #include <string>

    int main()
    {
        std::regex_traits<char> t;
        std::string str_alnum = "alnum";
        auto a = t.lookup_classname(str_alnum.begin(), str_alnum.end());
        std::string str_w = "w"; // [:w:] is [:alnum:] plus '_'
        auto w = t.lookup_classname(str_w.begin(), str_w.end());
        std::cout << std::boolalpha
                  << t.isctype('A', w) << ' ' << t.isctype('A', a) << '\n'
                  << t.isctype('_', w) << ' ' << t.isctype('_', a) << '\n'
                  << t.isctype(' ', w) << ' ' << t.isctype(' ', a) << '\n';
    }

Output:

    true true
    true false
    false false

Demonstrates a custom regex traits implementation of `lookup_classname()` / `isctype()`:

Run this code

    #include <cwctype>
    #include <iostream>
    #include <locale>
    #include <regex>

    // This custom regex traits uses wctype/iswctype to implement lookup_classname/isctype.
    struct wctype_traits : std::regex_traits<wchar_t>
    {
        using char_class_type = std::wctype_t;

        template<class It>
        char_class_type lookup_classname(It first, It last, bool = false) const
        {
            return std::wctype(std::string(first, last).c_str());
        }

        bool isctype(wchar_t c, char_class_type f) const
        {
            return std::iswctype(c, f);
        }
    };

    int main()
    {
        std::locale::global(std::locale("ja_JP.utf8"));
        std::wcout.sync_with_stdio(false);
        std::wcout.imbue(std::locale());

        std::wsmatch m;
        std::wstring in = L"風の谷のナウシカ";
        // matches all characters (they are classified as alnum)
        std::regex_search(in, m, std::wregex(L"([[:alnum:]]+)"));
        std::wcout << "alnums: " << m[1] << '\n'; // prints "風の谷のナウシカ"
        // matches only the katakana
        std::regex_search(in, m,
                          std::basic_regex<wchar_t, wctype_traits>(L"([[:jkata:]]+)"));
        std::wcout << "katakana: " << m[1] << '\n'; // prints "ナウシカ"
    }

Output:

    alnums: 風の谷のナウシカ
    katakana: ナウシカ

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 2018 | C++11 | the value of m was unspecified | matches lookup_classname()'s minimal support

### See also

lookup_classname | gets a character class by name (public member function)
do_is [virtual] | classifies a character or a character sequence (virtual protected member function of `std::ctype<CharT>`)
iswctype | classifies a wide character according to the specified `LC_CTYPE` category (function)

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